1. Field of the Invention
The present invention relates to a method of protecting a motor against an overload and a motor control device associated therewith, in which an overload state of the motor is detected and responsive thereto the motor is controlled its drive so as to be protected against overload.
2. Description of Prior Art
As well known hitherto, a motor drive control is effected for actuate the movable members in automobile, such as a powered seat or powered window. The motor is controlled its drive by a motor control device incorporating a microcomputer.
This motor control system has the likelihood of the motor's overload state when the movable members are forcibly stopped due to an externally applied forces, in which case, an over-current is flowed in the motor, causing overheat or damages to the motor per se. Thus, a safety aspect will be insufficient. Solution to this problem is to detect the overload state and quickly block supply of voltage to the motor, by means of an over-current detecting method, a motor revolution detecting method, or the like.
The over-current detecting method consists in detecting a terminal voltage of over-current detector resistance connected in series with the motor and if the voltage is raised, the overload state is determined.
The motor revolution detecting method consists in checking the pulse generating period and if the period is changed abnormally, the overload state is determined. In this method, the pulses are generated from a rotation sensor responsive to the revolutions of the motor.
However, with regard to the over-current detecting method, the voltage to the motor is insufficient because of its consumption in the over-current detector resistance, as a result of which a given revolution number, torque or other characteristics of the motor are not obtainable well. The current in the motor is sensitive to the ambient temperature and an increase of the temperature changes the motor current, which in turn changes the terminal voltage of the overcurrent detector resistance. Consequently, at the overload state, the torque of motor will be lowered below the normal torque range, thus preventing full use of motor characteristics. The motor is therefore limited its ranges for applications. On the other hand, as to the motor revolution detecting method, there is no such problem of motor characteristics since the voltage is constantly supplied to the motor without any resistances.
Nonetheless, in view of the fact that a battery is normally used as a power source for the motor in the automobile, the voltage supplied from the battery to the motor (motor voltage) is easily subject to changes according to the running speed of the automobile. This is understandable from FIG. 1, wherein the motor voltage (Eb) is not constant, and changeable at such degrees as indicated by Eb1, Eb2, Eb3 and Eb4. Such voltage variations are generally considered to fall within 9V to 16V. Within this voltage variation range, the motor is driven. Since the motor torque is in a proportional relation with the motor voltage, regardless of a given motor revolution number at No, the motor torque will be changed greatly with the voltage variations, as indicated by the designations Eb1 to Eb4 and T1 to T4. Thus, if the motor voltage amounts to the greatest value at Eb4 due to the overload state of motor in operation, the motor revolution number goes down below the set value (No), and torque turns excessively great at the Eb4. This apparently results in requiring greater rigidity and strength of drive mechanisms and movable members associated with the motor. Consequently, those mechanisms and members has to be increased in size and weight, with more complicated structure, thus making more annoying and troublesome the design works and assemblage of device.